This invention relates to mixed matrix gas separation membranes incorporating a molecular sieve material dispersed in a polymer.
The use of selectively gas permeable membranes to separate the components of gas mixtures is a commercially very important art. Such membranes are traditionally composed of a homogeneous, usually polymeric, composition through which the components to be separated from the mixture are able to travel at different rates under a given set of driving force conditions, e.g. transmembrane pressure, and concentration gradients.
A relatively recent advance in this field utilizes mixed matrix composite (MMC) membranes. Such membranes are characterized by a heterogeneous, active gas separation layer comprising a dispersed phase of discrete particles in a continuous phase of a polymeric material. The dispersed phase particles are microporous materials that have discriminating adsorbent properties for certain size molecules. Chemical compounds of suitable size can selectively migrate through the pores of the dispersed phase particles. In a gas separation involving a mixed matrix membrane, the dispersed phase material is selected to provide separation characteristics that improve the permeability and/or selectivity performance relative to that of an exclusively continuous phase polymeric material membrane.
U.S. Pat. Nos. 4,740,219, 5,127,925, 4,925,562, 4,925,459, 5,085,676, 6,508,860, 6,626,980, and 6,663,805, which are not admitted to be prior art with respect to the present invention by their mention in the background, disclose information relevant to mixed matrix composite membranes. U.S. Pat. Nos. 4,705,540, 4,717,393, and 4,880,442 and U.S. patent Publication Nos. 20040147796, 20040107830, and 20040147796, which are not admitted to be prior art with respect to the present invention by their mention in the background, disclose polymers relevant to permeable gas separation membranes. However, these references suffer from one or more of the disadvantages discussed herein.
Permselective membranes for fluid separation are used commercially in applications such as the production of oxygen-enriched air, production of nitrogen-enriched-air for inerting and blanketing, separation of carbon dioxide from methane or nitrogen for the upgrading of natural gas streams, and the separation of hydrogen from various petrochemical and oil refining streams. It is highly desirable to use membranes, such as MMC membranes, that exhibit high permeabilities, and good permselectivities in these applications.
MMC membranes that exhibit high permeabilities, and good permselectivities in some applications, especially hollow fiber applications, have proven problematic to the industry. Some MMC membranes suffer from poor performance due to problems dispersing the particulate molecular sieve material, particularly in polymers with low flexibility. Other MMC membrane processes use a high mass ratio of dispersed particles in the continuous phase, making the slurry difficult to process and increasing the brittleness of the membranes. Furthermore, some MMC membrane processes fail to teach how to prepare hollow fiber membranes using MMC suspensions. Some processes that do teach hollow fiber MMC membranes suffer from defects and macrovoids, which adversely affects attaining optimum selectivity as well as lowering the mechanical integrity of the fiber. Thus, many prior MMC membrane materials fail to provide a membrane with an optimum balance of high productivity and selectivity (particularly for the fluids of interest discussed above), and that are easily processed into a variety of membrane forms.
The fabrication of MMC hollow fiber membranes for gas separation modules is particularly problematic for the industry. Producing hollow fiber membranes typically involves extruding the nascent fiber through narrow channel extrusion dies, (spinnerettes) at very high shear rates. Such high shear conditions can impair microstructural stability of the heterogeneous composition and thus agglomerate and concentrate the particles so that a uniform dispersion is not maintained. The manufacture of MMC hollow fiber membranes also normally calls for axially drawing the nascent fibers to provide them with precise and uniform cross section dimensions. Drawing the nascent fibers as they emerge from the spinnerettes engenders stresses that can cause discontinuity at the interface between the dispersed phase particles and the continuous phase polymer within the mixed matrix. This contributes to the formation of macrovoids at the interface. Such macrovoids provide the gases migrating through the membrane the opportunity to bypass the active selective separation portions. Consequently, the desired high selectivity cannot be achieved. The formation of macrovoids also decreases the mechanical integrity and pressure capability of the MMC hollow fiber. Furthermore, the MMC selectivity enhancement effect can only be seen at low draw ratios/take-up speeds; unfortunately, macrovoid size and frequency increases as the draw ratio decreases.
It remains highly desirable to provide a MMC gas separation membrane having molecular sieve particles dispersed in a continuous polymer matrix that is macrovoid-free, can consistently yield a combination of higher flux and selectivity, and have sufficient flexibility to be processed on a commercial basis into a wide variety of membrane configurations, including hollow fiber membranes. It is also desirable that the membrane has sufficient strength to maintain structural integrity despite exposure to high transmembrane pressures.